The proposed program will deal broadly with cellular and molecular aspects of antigen-recognition by thymus-derived lymphocytes (T-cells) and with T cell regulation of two immune responses: induction of cytotoxic T lymphocytes and diversification of variable domains of antibodies during primary antibody responses. Antigen-recognition will be studied by determining whether products of the major histocompatibility complex restrict those T cells that help the development of cytotoxic T cells and whether the restriction is "learned" in the thymus during differentiation. Antigen-recognition molecules will be studied with anti-idiotypic antibodies, and the specifity of these antibodies will be analyzed with recombined light and heavy immunoglobulin chains. Idiotypy will also be analyzed with monoclonal antibodies produced by cell fusion.